Whereas passenger vehicles and light utility vehicles usually comprise a pedal power assisted hydraulic brake system and heavier utility vehicles comprise a pneumatic power brake system, wheeled vehicles that are provided for off-highway use, such as construction machinery and agricultural tractors, are fitted with a boosted hydraulic brake system, which is referred to here as a hydraulic power brake system. Because of the required high brake forces with this type of wheeled vehicle, the actuating units of the wheel brakes, such as the wheel brake cylinders and brake calipers, comprise a relatively large displacement volume, which can only be supplied with a power brake system. Because construction machinery and agricultural tractors usually comprise hydraulic working units or even a hydrostatic drive, a hydraulic power brake system can advantageously use the already present hydraulic pressure supply system.
In DE 195 46 056 A1 and DE 198 59 737 A1, hydraulic power brake systems of a motor vehicle are described, each of which comprises two brake circuits, a valve arrangement of an ABS control system and a valve arrangement of an ASR control. The inlet valves and the outlet valves of the valve arrangement of the ABS control system are each embodied there as 2/2-way solenoid valves. In the case of the valve arrangement of the ABS control system according to DE 195 46 056 A1, safety valves are embodied as non-return valves that are disposed parallel to the inlet valves. By contrast, the valve arrangement of the ABS control system according to DE 198 59 737 A1 comprises non-return valves or non-return valve functions that are integrated within the inlet valves for this.
Because of the directly electromagnetically operated inlet valves and outlet valves of the ABS control system, the known power brake systems are only suitable for passenger vehicles and lighter utility vehicles. With larger and heavier utility vehicles, especially with construction machinery and tractor vehicles provided for off-highway use, the actuating units of the wheel brakes, such as wheel brake cylinders and brake calipers, are dimensioned to be suitably large for producing the required brake forces, so that the displacement volume of the hydraulic oil to be moved in each of the actuating units during braking processes is often at least 15 cm3. For passing through and shutting off such large displacement volumes, relatively large solenoid valves with very high current drain of the suitably dimensioned magnetic coils are necessary. Because of the high inductivity of the magnetic coils, suitably implemented inlet valves and outlet valves would comprise long switching times corresponding to the slow build-up and decline of the magnetic fields and would only enable a low ABS control system frequency.
To avoid the disadvantage, in DE 10 2015 014 205 A1 a hydraulic power brake system is presented, with which the inlet valves and outlet valves of the ABS control system are embodied as hydraulic pressure-controlled switching valves. For actuation, either a 3/2-way solenoid valve or two 2/2-way solenoid valves may be associated with the inlet valves and outlet valves, respectively, via which the control inputs of the inlet valves and outlet valves can be alternately connected to a pressurized control pressure line or to an unpressurised return line. The control pressure lines branch off from a control pressure main line that is connected to a main brake line carrying a variable brake pressure.
Because the switching speed of the inlet valves and outlet valves of the ABS control system and hence the possible level of the ABS control system frequency reduces with reducing sound pressure, in DE 10 2015 015 472.9, which has not previously been published, the inventor proposed a hydraulic power brake system with which the inlet valves and outlet valves of the ABS control system are embodied as hydraulic or pneumatic pressure-controlled switching valves. So that the ABS control system is independent of the brake pressure that is controlled in the main brake line via the brake valve, the control pressure lines are connected via a control pressure main line to a main pressure line of a hydraulic or pneumatic pressure supply system.
The switching speed of the inlet valves and outlet valves and the pilot control valves of the ABS control system, and hence the possible level of the ABS control system frequency, are however also dependent on the temperature-dependent viscosity of the hydraulic oil that is used with hydraulic actuation. Therefore, in DE 10 2015 015 472.9 it is also proposed to provide a temperature sensor for detecting the oil temperature and a heating element for heating the available hydraulic oil. The temperature sensor and the heating element are preferably disposed on or in a storage container of the hydraulic pressure supply system.
To reduce the costs of manufacturing and assembly, the inlet valves and outlet valves and the pilot control valves of the ABS control system are usually disposed together in an ABS modulator block either for each vehicle unit (towing vehicle, trailer) or for each vehicle axle (front axle, rear axle). To protect against harmful ambient influences and mechanical damage, the ABS modulator blocks are attached to the frame of the vehicle at a suitable point very close to the actuating units of the wheel brakes. This necessarily results in greater distances between the hydraulic pressure supply system and the ABS modulator blocks, which are bridged by suitably long control pressure main lines. Hydraulic oil heated in the pressure supply system when needed therefore cools again in cold and humid weather conditions on its way to the ABS modulator blocks by thermal conduction and convection, so that the desired effect, namely to achieve a rapid response of the inlet valves and outlet valves of the ABS control system owing to low viscosity of the hydraulic oil, can fail to materialize or at least be highly attenuated.
It is the object of the present disclosure to propose a hydraulic power brake system of a wheeled vehicle of the aforementioned design type with improved operating characteristics of the ABS control system. Moreover, a method for ABS control of a hydraulic power brake system of this type will be presented.
The disclosure therefore concerns a hydraulic power brake system of a wheeled vehicle with at least one main brake line, in which a setpoint brake pressure can be controlled by a brake valve that is operated by a brake pedal, and from which at least one axle brake line or wheel brake line leading to an actuating unit of at least one wheel brake branches off, and with a valve arrangement of an ABS control system respectively comprising a hydraulic pressure-controlled inlet valve that is disposed between the main brake line and the respective axle brake line or wheel brake line, and a hydraulic pressure-controlled outlet valve that is disposed between the respective axle brake line or wheel brake line and an unpressurised return line, and at least one electromagnetically operated pilot control valve that is connected upstream to each inlet valve and outlet valve, wherein at least one temperature sensor for detecting the oil temperature and at least one heating element for heating hydraulic oil are provided, and wherein the inlet valves and outlet valves and the pilot control valves of the ABS control system are respectively disposed together in an ABS modulator block for each vehicle unit or for each vehicle axle.
To achieve the stated object, with the power brake system, it is provided that at least one temperature sensor for detecting the oil temperature and at least one heating element for heating the hydraulic oil that is used locally for control are disposed in each ABS modulator block. This enables in each case the local oil temperature of the hydraulic oil in the ABS modulator blocks close to the wheel brake to be detected and the hydraulic oil to be locally heated when needed, by activating the respective heating element and as a result to reduce the viscosity of the hydraulic oil. Possible cooling of the hydraulic oil in the path between a distant reservoir container and the ABS modulator blocks is compensated by increasing the temperature.
Because the switching speed of the inlet valves and outlet valves and hence the level of the ABS control system frequency substantially depends on the viscosity of the hydraulic oil that is used for control, in order to increase the accuracy of the temperature detection and the effectiveness of the heating of the hydraulic oil, it is preferably provided that the temperature sensor and the heating element are disposed close to the respective pilot control valves within the respective ABS modulator block.
A further increase in the accuracy of the temperature detection and the effectiveness of the heating of the hydraulic oil can be achieved by disposing a temperature sensor and a heating element are close to the associated pilot control valves in at least one ABS modulator block for each pair of inlet valves and outlet valves of the axle brake lines or the wheel brake lines.
The effectiveness of the heating of the hydraulic oil that is used for control can also be increased as a result of the fact that at least one ABS modulator block is provided with thermal insulation at least close to the pilot control valves that shields against the surroundings. The heat loss of the hydraulic oil into the surroundings by heat conduction and convection and the necessary heating power of the respective heating element is reduced by the thermal insulation.
The heating elements are preferably embodied as electrical heating bodies, each of which can be alternately connected to or disconnected from an electrical power supply of the wheeled vehicle via an actuatable electrical switch or a relay.
Alternatively, it can be provided that at least one heating element is embodied as an electrical heating body that can be alternately connected to or disconnected from an electrical power supply of the wheeled vehicle under self-control and depending on the prevailing temperature via a bimetallic switch that is integrated within or on a pilot control valve.
It is also possible, however, that the heating elements are embodied as oil heat exchangers, each of which can be alternately connected to or isolated from a cooling oil circuit of an oil pump of a hydraulic pressure supply system or a combustion engine or a driving gearbox or a working gearbox of the wheeled vehicle via an actuatable solenoid valve that is disposed in the feed or return.
Likewise, the heating elements can also be embodied as water heat exchangers, each of which can be alternately connected to or shut off from a cooling water circuit of a combustion engine of the wheeled vehicle via an actuatable solenoid valve that is disposed in the feed or return.
The disclosure also concerns a method for ABS control of a hydraulic power brake system of a wheeled vehicle comprising at least one main brake line, in which a setpoint brake pressure can be controlled by a brake valve that can be actuated by a brake pedal, and from which at least one axle brake line or wheel brake line leading to an actuating unit of at least one wheel brake branches off. Moreover, the power brake system comprises a valve arrangement of an ABS control system, which respectively comprises a hydraulic pressure-controlled inlet valve that is disposed between the main brake line and the respective axle brake line or wheel brake line, and a hydraulic pressure-controlled outlet valve that is disposed between the respective axle brake line or wheel brake line and an unpressurised return line. Further, the power brake system comprises at least one electromagnetically operated pilot control valve that is connected upstream of each inlet valve and outlet valve, and at least one temperature sensor for detecting the oil temperature and a heating element for heating hydraulic oil.
To achieve the method-related object for control of the hydraulic power brake system of a wheeled vehicle, it is provided that the local oil temperature of the hydraulic oil that is used for actuation of the inlet valves and outlet valves is measured by at least one temperature sensor that is disposed close to the pilot control valves of at least one pair of inlet valves and outlet valves of an axle brake line or a wheel brake line, and that at least one heating element that is disposed close to the same pilot control valves is switched on if the detected oil temperature has reached or fallen below a specified switch-on limit value TH_on, and is then switched off if the detected oil temperature has reached or exceeded a specified switch-off limit value TH_off, which is greater than the switch-on limit value TH_on.
The oil temperature of the hydraulic oil that is used for actuation of the inlet valves and outlet valves is thus detected close to the wheel brakes, and the respective hydraulic oil is heated locally when needed for reducing the viscosity. In this manner, the accuracy of the temperature detection of the hydraulic oil is increased on the one hand, and the required heating power for heating the hydraulic oil is reduced on the other hand.
In addition, according to a development of this method it is provided that the ABS control system is deactivated if the detected oil temperature T has reached or fallen below a specified operating limit value TABS_on, which is greater than a minimum limit value TABS_min for satisfying the legal requirements on the ABS control system (TABS_on>TABS_min) and is less than the switch-on limit value TH_on of the heating element (TABS_on<TH_on), (T≤TABS_on). Owing to the separation of the operating limit value TABS_on from the minimum limit value TABS_min, possible measuring inaccuracies during the sensor detection of the oil temperature T and a possible non-uniform temperature distribution within the respective ABS modulator block are taken into account, so that the ABS control system reliably satisfies the legal requirements in the activated state.
When the ABS control system is deactivated, an optical and/or acoustic warning signal is preferably output in the driver's cab of the wheeled vehicle in order to inform the driver of the non-availability of the ABS control system and to drive cautiously. The output of the warning signals can for example be carried out by permanent or flashing illumination of a suitably characterized warning light in the instrument panel of the driver's cab, by outputting a permanent or interrupted warning note by a loudspeaker and/or by written information on a display.